Alloy and method for treatment to produce spheroidal-graphite cast irons

ABSTRACT

THE TREATMENT OF IRONS FOR THE PRODUCTION OF SPHERIODAL-GRAPHITE CAST IRONS BY THE ADDITION OF AN ALLOY CONTAINING SILICON, MAGNESIUM, BARIUM AND NITROGEN WITH THE REMAINDER BEING IRON PLUS IMPURITIES.

United States Patent Ofice 3,717,456 Patented Feb. 20, 1973 3,717,456 ALLOY AND METHOD FOR TREATMENT T PRO- DUCE SPHEROIDAL-GRAPHITE CAST IRONS Jean-Claude Percheron and Louis Septier, Chedde, France, assignors to Compagnie Pechiney, Paris, France No Drawing. Filed Mar. 4, 1971, Ser. No. 121,178 Claims priority, application France, Apr. 16, 1970, 7013757 Int. Cl. C22c 37/04 US. Cl. 75-130 B 14 Claims ABSTRACT OF THE DISCLOSURE The treatment of cast irons for the production of spheroidal-graphite cast irons by the addition of an alloy containing silicon, magnesium, barium and nitrogen with the remainder being iron plus impurities.

This invention relates to an alloy for the treatment of spheroidal-graphite cast irons.

Since the discovery by Millis of the effect which magnesium has upon the shape and structure of the graphite precipitated in cast irons, considerable progress has been made in the manufacture of spheroidal-graphite cast irons.

It is now common practice to nodularize cast irons by means of alloys, often based on Fe-Si, containing magnesium as the nodularizing agent. The use of such alloys has the advantage, over the introduction of pure magnesium, of better distribution of the active element and far less violent reactions, whatever the method of introduction used.

By adjusting the quantity of magnesium to be introduced in known manner to the oxygen and sulphur contents of the cast iron and by working in the absence of anti-nodularizing elements (for example Bi, Pb, Ti), i.e. on untreated cast iron, the graphite is nodularized fairly easily in cases where it is merely required to obtain a ferrito-perlitic structure in the matrix of the hardened cast iron.

By contrast, problems arise where is is desired to produce a spheroidal-graphite cast iron with a ferrite matrix which is distinguished both by its outstanding machinability and its high ductility. This is because the additional graphiting required is inter alia a function of time while the nodularizing effect of the Mg disappears after about to minutes. According to one known proposal, which has been widely adopted, the iron is subjected to two successive treatments immediately prior to casting, namely to the nodularizing treatment referred to above followed by inoculation with graphiting agents whose main object is to eliminate all or some of the carbides and to reduce the rate at which the effect of the nodularizer disappears. The inoculants used are generally silicon alloys which can be doped with rare earths or with alkaline earths, for example, and which can be weighted through the incorporation of iron.

They are introduced as closely as possible to final casting, for example in the mold itself, inter alia by projection onto the walls of the mold when there is a danger of hardening, more particularly at those places where the casting comprises thin zones which solidify into a white and hence brittle structure.

Notwithstanding the precautions taken, the conventional two-stage treatment often produces unsatisfactory results. As a result, it has to be completed by a heat treatment of the solid metal, namely by annealing above the Ac point. It will be obvious to the skilled in the art that such annealing gives rise to surface faults which, in turn, produce such tensions, especially in the case of castings having zones of highly variable thickness, that permanent deformations occur.

It is an object of this invention to provide an additive alloy by which it is possible to obtain a spheroidalgraphite cast iron with an accentuated ferrite matrix, in which such results can be achieved in a simple treatment and in which such treatment can be achieved while the metal is in a crude cast state.

The use of the alloy, according to the invention, thus affords a number of advantages. It simplifies the handling of and reduces the losses of heat from the liquid pig iron. The losses of magnesium are reduced, thus allowing substantial quantities of this nodularizing element to be saved and much tighter control of its final content. It is known that magnesium stabilizes carbides thereby increasing the fragile/ductile transition temperature of the cast iron with the result that its content has to be adjusted to the minimum compatible with effective nodularization. On the other hand, it is no longer necessary to adjust the silicon content of the cast iron by the addition of expensive silicon-based alloys after it has been nodularized with magnesium. This is because, by virtue of the novel additional alloys, it is possible to adjust the silicon content in the blast furnace or in the cupola furnace by means of less expensive starting materials.

The additive alloy of the present invention is of the Fe-Si-Mg type and contains the Ba-N couple, the nitrogen being introduced, for example, by bubbling. The composition is as follows:

and nitrogen in a quantity such that the ratio by weight of Ba to N is between 5 and 50, the remainder being iron apart from the inevitable impurities introduced during its manufacture, more particularly Ca and Al, in a total concentration of less than 2.5%. The alloy is further characterized by a ratio by weight of MgzBa of 1 part by weight Mg to l-3 parts by weight Ba.

The various constituents of the alloy of the invention are balanced to produce not only a ferrite matrix, but also a uniformity in the size of the spheroids precipitated and a level of resistance to the formation of troublesome elements (especially carbide formers) which is reflected in the tolerance, during manufacture of the cast iron, of a recycling rate (introduction of scrap and returns) which can reach 50% and higher.

In its preferred composition, the alloy of this invention is characterized by the following:

Percent Si 45-50 Mg 7-11 Ba 4.5-6 N 0.4-0.6

With the remainder iron and the inevitable impurities of less than a total weight of 1.2%.

So far as the proportions are concerned, it has been found that the following represent the preferred ratios by weight:

Mg/Ba 2:0.2 Ba/N 10:2

between 0.03 and 0.06% by weight. In such instance, the

quantity of alloy to be used is such that the magnesium is usually present in a quantity of from 0.08 to 0.3% of the weight of the cast iron to be treated.

It is known (Schweizer Archiv, December 1964, page 367) that, although the introduction of nitrogen by bubbling can produce appreciable inoculation in spheroidal graphite cast irons, this technique does not result in the formation of a ferrite matrix in the crude cast state.

According to another prior publication (US. Pat. No. 3,177,072), cast irons can be nodularized with additive alloys of the Fe-Si-Mg-Ca-N type in which some of the nitrogen is present in the form of a suspension of calcium cyanarnide.

Unfortunately, the calcium associated with the aforementioned elements produces a less complete level of ferritization than that which can be obtained with barium using the alloys to which the present invention relates. This surprising result is illustrated in Tables 1 and 2 in which the letters have the following meaning:

A=treatment with an alloy according to the invention B-=substantially identical treatment except that the alloy contains Ca instead of Ba and then a quantity, equivalent to approximately 0.8% of the weight of the cast iron obtained, of an inoculating alloy which is a ferrosilicon with a silicon content of 75%.

After treatment, castings and test specimens are cast.

The three comparative treatments with alloys A, B and C are carried out on the same basic cast iron made in an induction furnace. Three batches of 600' kg. each are removed from the furnace for each of the treatments described.

In every case, the temperature of the cast iron during treatments A, B and C is between 1480 C. and 1520 C., the castings and the control specimens are produced at the end of ladle casting, the period of time elapsing between the treatment and casting of the specimens under examination was between 10 and 15 minutes.

Tables 1 and 2 give the results obtained from micrographic structural examinations and from mechanical tests on the crude cast metal.

TABLE I.MICROGRAPH1C EXAMINATION AND RESULTS OF ANALYSIS Percent of ferrite in castings in dependence upon the thickness or the castings Analysis or castings (e in mm.)

Ref. 0 Si Mn Mg Appearance of the graphite e= 50 e= e=5 A.-- 3. 6 2. 72 0. l8 0. 052 Perfectly spheroidal 95 90 85 B 3. 6 2. 63 0. l7 0. 040 spheroidal but with numerous pseudo-flakes- 80 6O C".-- 3. 6 2. 65 0.21 0. 042 Perfectly spheroidal 70 50 10 C=conventional treatment comprisin nodularization followed by inoculations.

Liquid cast iron is made in an acid cupola from a batch comprising 50% of new cast iron and 50% of scrap castings, being subsequently desulphurized with CaC by known methods.

The cast iron is then transferred to a holding-type induction furnace. Passage through this holding furnace enables the silicon content of the cast iron to be adjusted before treatment A and B so as to obtain, after treatment, a cast iron having a composition comparable to that of a cast iron which has been subjected to treatment of type C.

Treatments A and B comprise introducing, in sandwich" form, into the cast iron to be treated, a quantity of approximately 1.8% by weight of this cast iron of an alloy containing for treatment A:

Percent by weight Si 47.8 Mg 9.1 Ba 4.6

Fe: the remainder to 100%.

For treatment B: Percent by weight Si 47.6 Mg 9.2 Ca 5.3

Fe: the remainder to 100%.

Treatment C (according to the prior art) comprises introducing in sandwich form into the cast iron to be treated, first a quantity equivalent to about 1.8% by weight of this cast iron of a nodularizing Fe-Si-Mg alloy with 8.10% Mg content containing:

Si 47.5% by weight. Mg 9.3% by weight. Ba traces. Ca 0.2% by weight. Al 0.21% by weight. N traces.

Fe making up the remainder to 100% TABLE 2.MECHANICAL PROPERTIES OF STANDARD TEST SPECIMENS [Average of three test specimens] Tensile Elastic limit strength, Elongation, Brinell at 0.2% L.E. Rt, kg./ A percent, hardness,

Ref. in kgJmm. mm. 2 1 =5.65 So H.B.

EXAMPLE 1 2.250 kg. of a master alloy, representing the practice of this invention, containing, in addition to iron and the usual impurities, 48.2% by weight silicon, 9.1% by weight magnesium, 4.75% by weight barium and 0.4% by weight nitrogen, is introduced by means of a known type of plunger into a ladle containing 75 kg. of cast iron, containing carbon and silicon, maintained at a temperature of 1480" C.

The resulting cast iron has the following composition:

Percent C 3.6 Si 2.85 Mg 0.047

12 minutes after treatment, the castings and the control specimens (without treatment) show a precipitation of perfectly spheroidal graphite. No carbide is present in the castings, even in the thin zones. In the control specimen, the matrix has a ferrite content.

EXAMPLE 2 4 kg. of a master alloy of this invention containing, in addition to iron and the usual impurities, 47.4% silicon, 5.02% magnesium, 2.70% barium, 0.4% calcium and 0.30% nitrogen, are introduced by means of a plunger into a ladle containing 75 kg. cast iron at a temperature of 1510 C.

After treatment, the cast iron has the following composition:

Percent C 3.5 Si 2.56

12 minutes after treatment, the castings show a precipitation of perfectly spheroidal graphite. No carbide is present in the castings, even in the thin areas. In the control specimen, the matrix has a ferrite content of 85 to 90%.

EXAMPLE 3 13.2 kg. of a master alloy, containing 49.4% by weight Si, 8.8% Mg, 4.85% Ba, 0.32% Ca, 0.46% N, the remainder to 100% consisting of iron and impurities, are introduced by the so-called sandwich technique into a ladle containing 600 kg. of a cast iron prepared in an arc furnace from a batch comprising 40% of scrap iron and 60% of scrap castings at a temperature of 1480 C. (In the sandwich technique, the mother alloy is initially placed at the bottom of a treatment ladle, covered with scrap iron and then the cast iron to be treated is poured into the ladle.)

After treatment, the cast iron contains 3.55% C, 2.70% Si and 0.058% Mg.

14 minutes after treatment, micrographic examination of the castings reveals a perfectly spheroidal graphite structure, a 75% ferrite matrix and total absence of carbide even in the thin parts of the castings.

EXAMPLE 4 600 kg. of cast iron having a temperature of 1480" C. are treated by the sandwich technique with 25 kg. of a master alloy of the invention, containing 43.2% Si, 3.6% Mg, 15% Ba, 0.19% N, the remainder to 100% consisting of iron and impurities.

Casting is finished 12 minutes after the treatment.

All the castings obtained consist of spheroidal graphite cast iron. Examination reveals a carbide-free ferrite structure in the crude cast state.

The average composition is as follows:

Percent C 3.55 Si 2.47 Mn 0.12 Mg 0.051 Fe: the remainder to 100%.

EXAMPLE 5 Percent C 3.53 Si 2.54 Mn 0.18 Mg 0.041

Fe: the remainder to 100% Percent by weight Si 40-55 Mg 315 Ba 1.4-8.6

nitrogen in an amount to provide a ratio by weight of barium to nitrogen within the range of 5 to 50, the remainder being iron plus impurities.

2. An alloy as claimed in claim 1 in which the principal impurities are calcium and aluminum.

3. An alloy as claimed in claim 2 in which the total amount of impurities does not exceed 2.5% by weight.

4. An alloy as claimed in claim 1 in which the magnesium and barium are present in the weight ratioof 1 part by Weight magnesium to 1.3 parts by weight barium.

5. An alloy as claimed in claim 1 having the following composition in percent by weight:

Si 45-50 Mg 7-11 Ba 4.5--6 N 0.4-0.6

MgzBa 2i0.2 Ba:N 10:20

8. The method for treatment to produce spheroidal graphite cast irons comprising introducing into the Cast iron while in the molten state an alloy additive consisting essentially of 4055% by weight Si, 3-15% by weight Mg, 1.48.6% by Weight Ba, nitrogen in an amount to provide a weight ratio of Ba: N within the range of 5-50, the remainder being iron plus impurities.

9. The method as claimed in claim 8 in which the total amount of impurities is less than 2.5 by weight.

10. The method as claimed in claim 8 in which the principal impurities are calcium and aluminum.

11. The method as claimed in claim 8 in which the magnesium and barium are present in the weight ratio of 1 part by weight magnesium to 1-3 parts by weight of barium.

12. The method as claimed in claim 8 in which the materials are present in the amount of 45-50% by Weight silicon, 711% by weight magnesium, 4.5-6% by weight barium, 0.40.6% by weight nitrogen, the remainder being iron plus impurities.

13. The method as claimed in claim 12 in which the impurities do not exceed a total of 1.2% by weight.

14. The method as claimed in claim 8 in which the materials are present in the weight ratio of MgzBa of 2:02 and Ba:N of 10:2.

References Cited UNITED STATES PATENTS 2,676,097 4/1954 Strauss 75134 S X 3,177,072 4/1965 Kaess et al. 75l34 S 3,375,104 3/1968 McClellan 75l34 S X L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 75-134 S 

